Fusion SSTO airbreathing engine concept

In this thread i introduced my current addon project, a Single Stage to Orbit Airbreathing Vessel using Polywell Fusion Reactors.

Here is now a more detailed concept of the engine.

as you can see in the picture above the engine consists of the following parts:
1. MHD Extractor
This device is used at high mach numbers only. The inflowing airstream is ionized by an electron beam and then up to 50% of the kinetic energy is extracted using a hall and faraday type electrode setup together with a strong magnetic field. About 25% of the kinetic energy can be extracted as electric energy by the faraday setup, the other 25% are transformed into thermic energy of the inflowing airstream. Unfortunately this setup leads to a significant loss of total pressure (about 70%) but also slows the airstream down significantly to Mach numbers of 2-3.
That means the use of this device will only be helpful at Mach numbers where the inlets efficiency drops below 0.3 (approx. Mach 6-7) so the device is only used during atmospheric acceleration phase for the orbital insertion.

2. Ramjet Inlet
This slows the incoming airstream down to subsonic speeds by creating hypersonic shocks at the inlet flaps.

3. Heat Exchanger
This is used in two modes. The first mode involves cooling of the waste energy of the Fusion Reactors at lower Mach Numbers (up to ~M5). At higher mach numbers the temperature behind the inlet gets to high to dump heat into the airstream. The Fusion Reactors then have to be cooled regeneratively using the LH2 drive mass. In this flight regime LH2 is routed through the Heat Exchanger to cool the inflowing airstream to below 1500K to take thermal stress from the internal flow channel.

4. electric Fans
These are Fans driven by superconducting motors used for the initial flight phases (takeoff and climbout, then acceleration up to about M3). Once the Temperature at the Fan rises above ~600K the Fan Bypass Flaps are brought into the up position and the engine is used in Ramjet mode.
In the initial phase of the flight the Vessel is driven only by the Fans without using the QED engines to further heat the exhaust stream. In this mode the Direct exhaust flap is open and the QED Feed Flap closed. The flow is choked at the exhaust flap to limit Mach number at the Fan face to 0.5. After reaching an altitude of 8-10km the direct exhaust flap is closed and the QED Feed Flap opened to feed the airstream into the QED engine where it is heated and exhausted as hot plasma through magnetic nozzle of the QED engine.

5. QED engine
This is described in more detail in the papers of Robert W. Bussard. In short it uses the high voltage, moderate amps energy provided by the Fusion Reactors to power a Relativistic electron beam to heat the drive mass (air in airbreathing mode, LH2 in rocket mode). The drive mass is ionized and heated to temperatures up to 120000K and then electromagnetically confined and exhausted through a magnetic nozzle.

Engine modes:
1. Taxi, Takeoff, Climbout, Approach Landing
MHD Extractor is off, Heat Exchanger is used to dump Waste Heat of the Reactor into the airstream. The electric Fans provide a pressure rise of factor 4-5. The direct exhaust flap is opened and the exhaust provides approx. 150tons of Thrust at sea level. This mode is used until reaching an altitude of at least 8km where mode 2 can be used.

2. Accelerate to cruise speed
Direct Exhaust Flap is closed and QED Feed Flap opened. The airflow is fed into the QED engine, where it is heated and exhausted as hot plasma. Use of this mode is strictly discouraged below at least 8km altitude. The hot plasma exhaust will produce significant levels of Ozone which is not such a bad thing in the Stratosphere but nothing you would want at sea level.
The fan flaps remain open until the fan face temperature reaches ~600K. Beyond that the flaps are closed and the engine used in mode 3.

3. Ramjet cruise mode.
Fan flaps are closed and the engine used as a ramjet feeding Air directly into the QED engines. The high inlet temperatures at higher mach numbers limit cruise flight to about Mach5 where just enough waste heat from the fusion reactor can be dumped through the heat exchanger without thermally overstressing it.

4. Orbital insertion airbreathing acceleration part.
In this mode the Vessel accelerates to speeds well above Mach 10 using mainly air as drive mass. However cooling of the fusion reactor waste heat cannot be done using the heat exchanger because of to high inlet air temperatures. Instead a bit of onboard LH2 is used as drive mass too which is routed through the reactor, inlet and heat-exchanger prior to being brought to the QED engine. This ensures cooling of the reactor and inlet and cools the airstream in the heat exchanger to temperatures below 1500K. At airspeeds above Mach 7 the Ramjet inlet become significantly inefficient leading to a high total pressure loss. At this point the MHD Extractor is activated, slowing the incoming air to Mach 2-3 thereby heating it as well as extracting a bit of electrical energy that is used for the Electron Beams to ionize the incoming air in front of the MHD Extractor.
At a certain airspeed (around Mach 10-15) the LH2 massflow needed to cool the Inlet and Airstream will significantly exceed the massflow of airstream making the airbreathing mode less efficient than pure rocket mode. The vessel is then brought out of the denser atmosphere in a pitchup maneuvre bringing it into a parabolic flight trajectory. The engine is then switched to mode 5 to accelerate to orbital speeds.

5. Rocket mode
In this mode the complete airbreathing inlet parts are retracted into the airframe and the QED engines are fed by onboard LH2. In this mode the ISP can be adjusted between 1500s and ~6000s giving a thrust of about 150tons at 1500s ISP and significantly lower thrust at higher ISPs.

I once saw a show on the old Discovery Wings Channel about Project Pluto. It was absolutely fascinating. They had a vast fleet of air compressors and compressed air tanks along with air heaters in order to supply air at the pressure & temperature that the reactor would see in flight. They got as far as planning a flight test. One idea was to attach the missile to a cable anchored to the ground to ensure the missile didn’t fly outside the test range. Imagine a 15ton, nuclear powered tether-ball! At this point cooler heads prevailed, ICBMs entered service, and the project died.

Correct me if I'm wrong, but this isn't just airbreathing, it's atmosphere breathing. Meaning it could work on Mars (perhaps, not much atmo there) or Titan? I had a similar idea, not as well thought out, for a Titan glider. After all, the air isn't being used for combustion, so O2 content isn't required.

@pattersoncr: Oh well a bomb just sits there until it blows up but with a Mach 3 cruise missile flying around with essentially unlimited fuel you never know what could happen i guess.

@Tommy: Yes you are right its essentially a gas-heating engine. I guess the Mars atmosphere would be a bit thin but Titan should work. Maybe also Venus if the ship could somehow cope with the high ground pressures and sulfuric acid.

Funny stuff i just tested this.
Mars: Not really practical.. next to no thrust from the fans, even with QED engines on the thrust is not enough to get the ship in motion. So you need rocket mode for takeoff. Minimum takeoff speed is then at around M3 where i would expect the landing gear to be just debris. You can then use the airbreathing mode to some extent up to ~M7 then the massflow gets too low to generate sufficient thrust. So its rocket mode from there again.

Titan: Now that one is nice. Orbit velocity at around M5.5 The engines would not be very effective but give sufficient thrust to get to orbit with only the last bit from M4.5 to 5.5 needing mode 4. So you could get to orbit with next to no LH2 used -> huge payload possibilities.

Venus: A real showstopper. Unfortunately the maximum temperature limit for the fans is 600K for now so on Venus they wouldn't work at 750K surface temperature. Another problem is that currently i have the fan motors limited to 300MW power which is not nearly enough on venus to reach a pressure ratio of > 2:1 to be able to simulate a choked nozzle condition. I might try some other way to simulate thrust from the fans but i guess it would all feel more like driving a submarine than an airplane.